1. Field of the Invention
The present invention relates to an evaporated fuel treatment device of an engine.
2. Description of the Related Art
Known in the art is an internal combustion engine provided with a canister for temporarily storing evaporated fuel generated in a fuel tank and a purge control valve for controlling the amount of purge of the fuel vapor to be purged from the canister to the inside of an intake passage, which stores in advance the relationship between the amount of evaporated fuel absorbed in the canister and the concentration of the vapor in the intake passage when the purge action is being performed, finds the initial concentration of vapor immediately after the start of the purge from the amount of deviation of the air-fuel ratio, uses the above relationship stored in advance to find the initial amount of absorbed evaporated fuel of the canister from the initial concentration of vapor, then finds the amount of reduction of the amount of absorbed evaporated fuel of the canister per unit time from the initial concentration of vapor, then uses the relationship stored in advance to predict the concentration of vapor from the reduced amount of absorbed evaporated fuel of the canister, finds the amount of reduction of the amount of absorbed evaporated fuel of the canister per unit time once again from the predicted concentration of vapor, then uses the relationship stored in advance to predict the concentration of vapor once again from the reduced amount of absorbed evaporated fuel of the canister, and corrects the amount of fuel supplied based on the predicted concentration of vapor so that the air-fuel ratio becomes a target air-fuel ratio (see Japanese Unexamined Patent Publication (Kokai) No. 5-248312).
That is, in transitional operation, sometimes the air-fuel ratio fluctuates despite the concentration of vapor not changing. If however the concentration of vapor is calculated based on the amount of deviation of the air-fuel ratio, it is judged that the concentration of vapor has changed even in such a case. If the concentration of vapor is updated at this time, the air-fuel ratio will conversely fluctuate. Therefore, in the above internal combustion engine, it is assumed that the amount of absorbed evaporated fuel of the canister gradually falls after the purge action has started and, based on this assumption, it is predicted that the concentration of vapor gradually falls along with the elapse of time and the amount of evaporated fuel is corrected based on this predicted concentration of vapor so that fluctuation of the air-fuel ratio is suppressed.
When for example the temperature of the fuel is high, however, a large amount of evaporated fuel is absorbed in the canister when the purge is temporarily stopped and therefore the amount of absorbed evaporated fuel of the canister increases. In the above internal combustion engine, however, even in this case, the amount of absorbed evaporated fuel of the canister is considered to gradually fall. Since the predicted value of the concentration of vapor also is made to gradually fall, when the purge is restarted, the predicted value of the concentration of vapor deviates considerably from the actual concentration of vapor and therefore the problem arises of a considerable fluctuation in the air-fuel ratio.